Mali's Islamist rebels have smashed the entrance of a 15th century Timbuktu mosque, escalating a campaign of destruction of the city's cultural treasures despite threats of prosecution for war crimes.

Some residents sobbed as the Islamist militants broke down the 'sacred door' of one of the northern Malian city's three ancient mosques after they wrecked seven tombs of Muslim saints over the weekend.

Video footage obtained by AFP shows turbaned men chanting “Allahu Akbar” (God is great) while smashing a mausoleum with pick-axes in a cloud of dust, the mud-brick tomb showing gaping holes in the side with rubble piling up alongside it.

The Organisation of Islamic Cooperation condemned the destruction, saying in a statement the sites were "part of the rich Islamic heritage of Mali and should not be allowed to be destroyed by ...bigoted extremist elements."

Ansar Dine (Defenders of Faith) believe the shrines to be idolatrous and have threatened to destroy any mosques housing the remains of the ancient saints, prompting an outcry from government and the international community.

The door on the south end of the mosque has been closed for centuries due to local beliefs that to open it will bring misfortune.

According to the website of the UN cultural agency (UNESCO) Sidi Yahya is one of Timbuktu's three great mosques and was built around 1400, dating back to the city's golden age as a desert crossroads and centre for learning.

The fabled city, which became a metaphor for a mythic, faraway place, is considered one of the centres from which Islam spread through Africa in the 15th and 16th centuries.

Ansar Dine began their campaign of destruction after UNESCO put Timbuktu on its list of endangered world heritage sites.

Mali in a state of chaos

In a matter of months Mali has gone from one of west Africa's most stable democracies to a nation gripped by deadly chaos.

A 22 March coup eased the way for Tuareg separatist rebels -descendants of those who founded Timbuktu in the fifth century -to carry out the armed takeover of an area larger than France they consider their homeland.

However, the previously unknown Ansar Dine group fighting on their flanks seized the upper hand, are openly allied with Al-Qaeda in the Islamic Maghreb, and have since pushed the Tuareg from all positions of power.